I played Sunbreak is multiplayer so I can only speak to that experience; it's definitely tough, Master Rank monsters can cart you in seconds if you're not careful, but as someone who generally struggles with very difficult games, I found it surmountable. 

Blood and Wine is easily my favourite of all time. It’s also my favourite part of Witcher 3.
Although, the wedding sequence in Hearts of Stone is the my favourite chapter of the game. So, Hearts of Stone would have to be on my list, too.

Most of the rest of my favourite expansions are probably in Paradox Games.

Brood War was mentioned above, that was a great one.

Torna was another great one.

Going back a long time ago to an era known as the 1990s:
Sid Meier’s Civilization 2: Scenarios - which added macros/scripting features and a bunch of scenarios; and Fantastic World which expanded on those. The mod scene exploded after the release of these.
Warcraft 2: Beyond the dark portal was another major expansion that acted as a sequel to Warcraft 2 in all but name.

My list right now:

1. Blood and Wine
2. Hearts of Stone

3. Brood War

4. Torna

5. Civ 2 Scenarios

6. Civ 2 Fantastic Worlds

7. StarCraft Brood War

8. Crusader Kings 2: Old Gods, Holy Fury, Way of Life, Monks and Mystics, Charlemagne (I realize this is kind of cheating, but I like them all fairly equally).

9. Stellaris: Utopia

10. Mario Kart 8 Switch.

In no particular order.

Mario Kart 8
New Super Luigi U
Mass Effect 3: Citadel
Breath of the WIld Ballad of the Champions or whatever it was called.

1. Everquest: The Ruins of Kunark
2. Everquest: The scars of Velious
3. Everquest: Lost Dungeons of Norrath
4. Everquest: Omens of War
5. Baldur's Gate: Tales of the Sword coast
6. WoW: Burning Crusades
7. WoW: Wrath of the Lich King
8. Burnout Paradise: Big Surf Island
9. DriveClub: Bikes

Then there is No Man's Sky. It's still giving me new content for the original disk copy, and all in VR now. (I did upgrade to digital in a sale years ago, still have the original day 1 disk)

Cannot say I have 10, but Breath of the Wild was pretty full back in the day.

I feel like one of the Diablo games had one too that was nice, but that has been years.

In no particular order, here are a few gems:

• Age of Empires II: The Conquerors (2000) - AoE2 was already one of the better RTS games ever made, and this expansion built on it swimmingly. You had a bunch of quality of life changes, new visual elements such as snowy and tropical terrain, improved AI, an expanded tech tree, new maps, new campaigns, five new factions, and the new Uniquer Technology mechanic. There's a reason why the game survived for over a decade without support.
• Fallout: New Vegas - Old World Blues (2011) - Basically an entire Fallout game in miniature, complete with interesting quests, a decent story and roster of characters, new items/enemies/perks, and The Toaster.
• Mario Kart 8's content packs (2014 & 2015) - $12 for 16 high quality new tracks with original graphics and music, 6 new characters, 12 kart parts, and 200cc? This was a heck of a bargain. As a bonus, the Animal Crossing pack came out weeks early, something that hardly ever happens in this industry.
• Shovel Knight: Specter of Torment (2017) - A new campaign and delightful set of mechanics alongside remixed music to match the new tone.
• Sonic & Knuckles (1994) - This one is a bit arguable, but I'm including it here because buying this let you add new content to Sonic 2 and Sonic 3.

Should make a poll based on comments.

MK8 and it isn't even close. It doubled the size of the game. And the original Dark Souls DLC deserves some love. Artorias was awesome.

Sim City 4 Rush Hour.

Exactly, if you have extra people to take aggro it reduces difficulty a lot even though the bosses health doubles for every person. The NPC characters help too but they aren't great at being tanks. Some of the monsters are extremely aggressive and in your face like that one made of stone, absolutely brutal to fight solo and another problem that I hit onto is the best effects like affinity are on gear with shitter stats so building becomes an issue cause they hit so damn hard. It's doable solo but I can't imagine doing the end game where I seen it's like one good hit and your dead or very near it. Great expansion regardless though, easily the most content filled DLC ever with 100 hours main and side content and 330 hours for a completionist on How long to beat, which is way more than the base game. 

I haven't tried MK8 aside from a few local multiplayer races but Monster Hunter Sunbreak is larger than the base game by two dozen hours. So it's not the only DLC that does that. Iceborne for MHW is also pretty massive and War of the chosen and the DLC pack changes the game so much it might as well be a sequel. Also, MMO's would win this by metric, they double the game size everytime they release as well as adding new features in every aspect of the game and continue to expand them between releases. By the time an FF14 expansion is fully expanded it's wildly large in scope. 

I might put together a poll for most content filled DLC, yeah.